After years of forecasting gloom and doom for stocks only to watch them surge, Marc Faber is sounding the alarm as loud as ever.

Faber, editor of The Gloom, Boom & Doom Report, believes that stocks in the U.S. and in many places around the globe are in a central bank-fueled bubble. And while he can't put a time on when that perceived bubble will pop, he prognosticates that once it does, the outcome will be horrifying.

"For the last two years, I've been thinking that U.S. stocks are due for a correction," Faber said Wednesday on CNBC's "Trading Nation." "But I always say a bubble is a bubble, and if there's no correction, the market will go up, and one day it will go down, big time."

"The market is in a position where it's not just going to be a 10 percent correction. Maybe it first goes up a bit further, but when it comes, it will be 30 percent or 40 percent minimum!" Faber asserted.

A 40 percent decline from Wednesday's close would take the S&P 500 to 1,264, a level that hasn't been seen since the early days of 2012.

Faber says low yields and stimulative central bank policies around the world have led to a condition in which "all assets are grossly overvalued … and eventually this will unwind and cause some problems."

Despite his massively bearish call, Faber said he's "not short the market yet," since he doesn't know how high stocks could go in the interim. Still, he makes clear that "I'm not interested to buy momentum, I'm interested to buy value."

And recent market action could indicate that even the momentum has come to an end.

"Look at the market since November of last year to now. The market is up 2 percent. It hasn't done much, and a lot of stocks are breaking down. I don't think that the market is in a healthy condition."

Faber's calls have slowly become more dramatically bearish over time, as stocks have ticked higher. For instance, in November 2012, Faber predicted that "corporate profits are going to disappoint, the global economy will hardly grow next year or even contract, and that is the reason why stocks, from the highs of September [2012] of 1,470 on the S&P, will drop at least 20 percent."

That call has proved less than prescient. But the target implied by that call—1,176—is not too far below his current implied target of 1,264.

Of course, for market forecasters, accuracy is probably more important than consistency.